Bi 3.25La 0.75Ti 3O 12 (BLT) ferroelectric thin films (<100nm) were deposited on Pt/Ti/SiO 2/Si substrates using chemical solution methods. The ultraviolet-visible ellipsometric spectra of the BLT thin films have been investigated in the photon energy range of 2～4.5eV. Based on the classical optical functions of the dielectrics and five-phase structure model, the optical constants of the BLT thin films in the transparent and absorption region, the surface roughness, the thicknesses of the BLT thin films and the interface layer between the films and substrates were simultaneously obtained by the fitting. The dispersion of the refractive index in the transparent region fitted well to a single-term Sellmeier relation. Finally, a 3.96ev direct band gap of the Bi 3.25La 0.75Ti 3O 12 thin films was obtained employing Tauc's principle.